1. Field of the Invention
This invention relates generally to the manufacture of high performance semiconductor devices. More specifically, this invention relates to a systematic approach to an experimental design for large, complex systems. Even more specifically, this invention relates to a method for systematically designing experiments when prior knowledge of the many factor and multiple responses is expressed as a network of cause-effect relationships.
2. Discussion of the Related Art
The design of a new semiconductor device and the process for manufacturing the new semiconductor device has three phases: the development phase during which processing alternatives are still under evaluation and the nominal process targets continue to be tuned; the pre-production phase during which the process targets are more-or-less set, processing experience is acquired, and appropriate tolerance windows are determined; and the production phase during which both the process target and tolerance windows are more-or-less fixed, and the full resources of the manufacturing line are committed in volume. The present invention focuses on the pre-production phase during which process targets are substantially set, but process experience is to be accumulated, and during which appropriate tolerance windows need to be determined.
The pre-production phase plays an essential role in managing the manufacturing risk factor, providing a time period for determining problematic and challenging process steps, for investigating the range over which product can be manufactured successfully, and for reliability stress testing. The scale of current semiconductor manufacturing processes magnifies all of the issues, for example, contemporary semiconductor processes have 300–400 value added steps, any of which is a source of poor quality and/or reliability. A cost of a single test batch can exceed one quarter million dollars. A delay in market entry equal to one cycle of learning (the production time required to make one batch) approaches two orders of magnitude more.
Competitive pressures have provided strong incentives to keep pre-production costs to an effective minimum. In the statistics literature, it is well recognized that appropriately designed experiments more fully characterize processes than, for example, multiple repetitions of the nominal process. The pre-production assessment of a new semiconductor manufacturing process typically involves many factors and can be from 30–50 and sometimes more, multiple blocks, typically from 5–15 blocks, and several responses, typically from 3–8 responses. One practical constraint is for each experimental block to be self-contained, in the sense that each block supports an analysis without necessarily requiring results from other blocks. A complementary goal has the entire ensemble of experimental blocks covering the process space well. Subject matter expertise is both available and desirable and can be organized as a network of likely cause-effect relationships.
The present invention thus presents a systematic approach to the pre-production problem, including objectives, constraints, overarching model, blocking structures, split and skew factors and self-containing blocks.